U.S. patent number 3,907,614 [Application Number 05/446,550] was granted by the patent office on 1975-09-23 for bainitic ferrous alloy and method.
This patent grant is currently assigned to Bethlehem Steel Corporation. Invention is credited to Bruce L. Bramfitt, Arnold R. Marder.
United States Patent |
3,907,614 |
Bramfitt , et al. |
September 23, 1975 |
Bainitic ferrous alloy and method
Abstract
This invention relates to an as-worked bainitic ferrous alloy
and to a novel method of processing same to obtain optimum strength
and toughness. More particularly, this invention is directed to the
hot working cycle of a ferrous alloy characterized by an I-T
Diagram or "S" Curve having a double nose or a pearlite
transformation knee of the beginning curve above a broad bainitic
bay region. Such an alloys is heated to an austenitizing
temperature of about 1,500.degree. to 2,200.degree. F., and
subjected to a plurality of working operations at successively
lower temperatures, where the final working operation is conducted
after the beginning of the austenite transformation to bainite and
before the complete transformation thereof.
Inventors: |
Bramfitt; Bruce L. (Bethlehem,
PA), Marder; Arnold R. (Bethlehem, PA) |
Assignee: |
Bethlehem Steel Corporation
(Bethlehem, PA)
|
Family
ID: |
26980687 |
Appl.
No.: |
05/446,550 |
Filed: |
February 27, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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316962 |
Dec 20, 1972 |
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Current U.S.
Class: |
148/334; 148/335;
148/336; 148/337 |
Current CPC
Class: |
C21D
8/00 (20130101) |
Current International
Class: |
C21D
8/00 (20060101); C22C 038/12 () |
Field of
Search: |
;148/36,12F |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Stallard; W.
Attorney, Agent or Firm: O'Keefe; Joseph J. Noll; William
B.
Parent Case Text
This is a division of application Ser. No. 316,962, filed Dec. 20,
1972.
Claims
We claim:
1. An as-worked bainitic ferrous alloy consisting essentially, by
weight, of about 0.03 to 0.65 percent carbon, a minimum of about
0.25 percent molybdenum, an addition of at least one element
selected from the group consisting of boron, manganese, nickel, and
chromium, with the balance essentially iron, characterized by a
crystallographic texture dominated by a (111) ?110! grain
orientation and a microstructure of fine, elongated grains, with a
lath-like substructure, where said lath-like substructure is
characterized by the presence of fine carbides at the lath
boundaries and dispersed within the laths.
2. The alloy according to claim 1 wherein the carbides range in
size between about 0.01 to 0.03 .mu.m.
3. The alloy according to claim 1 wherein the carbon is present in
an amount between about 0.10 to 0.50 percent.
4. The alloy according to claim 1 wherein the intensity of said
crystallographic texture ranges from 2.5 to 3.5 times random as
measured by the nine-plane, inverse pole figure technique on the
plane perpendicular to the working direction of said alloy.
Description
BACKGROUND OF THE INVENTION
The invention, to be described in detail hereinafter, is directed
to an as-worked bainitic ferrous alloy, and to a process including
the thermomechanical treatment thereof. In general terms, bainite
has been metallurgically defined as one of the transformation
products of austenite. Upon cooling the austenite, transformation
to bainite occurs over the temperature range of about 1,000.degree.
to 450.degree. F. The microstructure differs from pearlite, a high
temperature austenite transformation product, in that it is
acicular in nature. The last generally recognized transformation
product is martensite. Transformation to martensite must be
directly from austenite, i.e. without prior transformation to
ferrite, pearlite or bainite. Unlike the transformation to pearlite
or bainite, it is not time dependent, but occurs almost instantly
upon cooling.
Over the recent years, the times, temperatures and rates of
transformation of various steels have been determined and plotted -
the result being a series of diagrams known as isothermal
transformation diagrams, TTT diagrams, or S-curves. One current
publication offering I-T and C-T or continuous cooling diagrams for
standard steels, as well as modified versions thereof, is the Atlas
of Isothermal Transformation Diagrams by U.S. Steel, 1963. It will
be appreciated that the temperature and range noted above is not
limiting but merely illustrative of a typical steel. A quick review
of an I-T diagram for a given steel will give a closer reading of
said steel. Nevertheless, while they are close, such diagrams are
not precise as to exact times and temperatures.
For example, the I-T diagrams were developed by studying the
transformation behavior of a steel at a series of temperatures
below the A.sub.1 critical temperature, by quenching small samples
to the desired temperature in a liquid bath, allowing them to
transform isothermally and following the progress of the
transformation metallographically. From this, the curves could be
drawn. Very generally, this procedure included heating, quenching,
holding, and cooling. It has since been determined that processing
variations such as working or rolling affected the actual time and
temperature of transformation, particularly the pearlite
transformation. More specifically it was discovered that the
inclusion of an austenite working step changed the transformation
kinetics of the steel by shifting the curve to a higher
temperature. This phenomena has been confirmed by Y. E. Smith in an
article in Metallurgical Transactions, V. 2, June 1971.
Nevertheless, with the apparent limitations, the I-T diagram
remains as a significant factor in selecting the proper ferrous
alloy and for predicting results when subjected to the process
herein.
With this background, consideration may now be given to certain
aspects of prior art practices. Thermomechanical treatment of steel
has long been considered as a means of improving mechanical
properties. Several practices have been developed with the most
well known of these being "ausforming". This is identified and
shown in the FIGURE, where the practice is schematically
illustrated superimposed on a typical I-T curve of a bainitic
ferrous alloy. "Isoforming" is another practice where the ferrous
alloy is worked as it isothermally transforms to pearlite. This,
along with standard and controlled hot rolling, is illustrated in
the FIGURE in the manner of ausforming.
Variations to certain of these practices have been developed as
evidenced by the following U.S. Pat. Nos. 2,717,846 - 3,215,565 -
2,240,634 - 3,303,061 - 3,340,102 - 3,444,008 - 3,453,152 and
3,645,801. While some of said patents treat alloys susceptible to
processing by the method herein, none of them individually or in
combination teach the present concept of subjecting the alloy in an
austenitized condition to a plurality of working operations,
preferably at least five, where the final working is concluded
after the initiation of the austenite transformation to bainite and
before the complete transformation thereof.
SUMMARY OF THE INVENTION
This invention is directed to an as-worked bainitic ferrous alloy
and to a procedure involving the thermomechanical treatment of the
said alloy. More particularly it relates to a process of heating
said alloy to an austenitizing temperature between about
1,500.degree. to 2,200.degree. F. and subjecting the alloy to a
plurality of working operations as it cools from said austenitizing
temperature. While the final working should be conducted during the
transformation of the austenite to bainite, the initial working
should begin while the alloy is above 1,500.degree. F. By this
procedure, an optimum combination of strength and toughness, in the
as-worked condition, is produced. Metallographically, the processed
alloy is characterized by fine grains and cells, with very fine
uniformly dispersed carbides.
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE represents a typical I-T curve of a bainitic alloy
susceptible to treatment by the process herein, with a number of
schematic representations of thermomechanical treatment cycles
superimposed thereover, comparing the present process to that of
four prior art processes.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
In the preferred practice of this invention, a bainitic ferrous
alloy characterized by an isothermal transformation diagram having
the pearlite transformation knee of the beginning curve laterally
displaced to the right of the lower temperature bainitic
transformation knee, is heated to an austenitizing temperature
between about 1,500.degree. to 2,200.degree. F., preferably at
least about 1,600.degree. F., and subjected to at least two working
steps, preferably at least five, to reduce the cross section
thereof, with the final of said steps occurring within the bainite
transformation region.
Without intending to unduly limit the invention, a number of
ferrous base alloys having the foregoing characteristic are listed
with nominal compositions in Table I, by weight percent:
TABLE I
__________________________________________________________________________
Baintic Ferrous Alloy C Mn Mo Cr Ni V Si Al B Cu
__________________________________________________________________________
A .39 .56 .74 3.53 B .23 .82 .53 1.22 .22 C .40 .78 .53 1.25 .22 D
.33 .84 1.07 1.05 .26 E .27 .84 .90 .73 .60 .11 F .25 .88 .88 .73
.59 .23 G .6 .45 1.52 3.33 H .55 .60 .19 1.03 .36 J .42 .78 .33 .80
1.79 K .62 .64 .32 .60 1.79 .67 L .59 .89 .22 .64 .53 M .57 .82 .26
1.07 1.16 N .55 .83 .48 1.01 1.15 P .51 .73 .45 .99 2.75 Q .41 .57
.36 1.57 1.26 R .46 .79 .18 .77 .91 .0021 S .15 .92 .46 .50 .88 .06
.0031 .32 T .39 .89 .50 .95 .88 .03 .48 .002 U .44 .79 .54 2.10 .06
1.63 V .26 .77 1.00 .95 .04 .25 W .035 .79 .97 .95 .02 .24
__________________________________________________________________________
Each of said bainitic alloys A-W are characterized by a double nose
I-T Diagram where the upper or pearlitic nose is laterally
displaced to the right of the lower nose or region of bainitic
transformation. For all of these alloys the end or tip of the
pearlitic nose is somewhere between 1,200.degree. and 1,350.degree.
F. In other words, by the proper chemistry selection, i.e.
balancing isothermal transformation retarding additions, and
hardenability additions, it is possible to select an alloy
possessing sufficient transformation time delays to permit working
and cooling from the austenitizing temperature past the pearlitic
nose, while permitting the bainitic transformation.
Returning now to the chemistry of the exemplary alloys of Table I,
it will be noted that four elements (C, Mn, Mo and Cr) are
conspicuous by their presence in all but two alloys. All of said
elements are present in the alloys for their effects on
hardenability, and the overall effect in retarding isothermal
transformation. The hardenability of a ferrous alloy is governed in
large part by its chemical composition, and each addition or
element present in the alloy affects hardenability to some degree.
Much of this is known so it will suffice to say that the degree
differs between additions, and between varying sub-ranges within a
broad range of elemental addition.
In the present invention, carbon, while inherently present in
steel, should be present in an amount sufficient to yield
precipitated carbides upon final cooling. Generally, it is
desirable to include from 0.10 to 0.50 percent, by weight, in the
alloy, but a broader range of 0.03 to 0.65 percent, by weight, may
be used with success. As stated previously, all elemental additions
affect hardenability. This may be extended to say that all
elemental additions, to a large or small degree, are effective in
retarding isothermal transformation. For instance, a number of
elemental additions, such as Mo, Cr, V, Si, Ti and Cb (identified
for convenience as Group I elements), are effective in pushing the
pearlite nose to the right. One of the most effective of these
additions is molybdenum. However, the latter additions should be
distinguished from some other elemental additions, such as Mn, Ni,
B, N and Co, which act to shift the entire "S" curve to the right;
these may be identified as Group II elements. Thus, while one or
more Group I additions are desirable, it is preferable that the
alloy contain at least 0.25 percent by weight, molybdenum. But,
when a lesser amount is used, it is desirable to use at least 0.75
percent, preferably, 1.00 percent, by weight, chromium. These
elements (Cr and Mo) and the others of Group I have been observed
and are classified as ferrite stabilizers, and as noted above, all
of said Group I elements retard the pearlite transformation, i.e.,
move the pearlite nose further to the right.
Considering now the procedural steps of this invention, it begins
by heating a bainitic ferrous alloy to an austenitizing temperature
of about 1,500.degree. to 2,200.degree. F. where the first working,
such as rolling, should start. Several reductions or passes,
preferably at least four, on the order of about 10 percent minimum
for each reduction, should occur between the starting temperature
and the onset of the bainitic transformation or the B.sub.s
temperature which occurs in the range of 900.degree. to
1,100.degree. F. This sequence causes grain refinement of the
austenite by repeated deformation and recrystallization. Additions
of carbide forming elements may aid in the refinement of austenite
grain size by precipitation in the austenite. At the lower
temperatures, the recrystallization of austenite is retarded and
the deformation causes substructural and textural development in
the prior austenite grains. The further and final aspect of this
procedure involves at least one additional pass below the bainitic
start temperature to provide additional dislocations which act as
nucleation sites for carbide precipitation. The latter is necessary
to form numerous, fine precipitates which increase strength and
improve notch toughness. Finally, this last reduction, in
combination with the prior treatments, provides an optimum
structure, i.e., fine grains and cells with very fine, uniformly
dispersed carbides, and an optimum crystallographic texture for
improved properties.
More particularly, the microstructure of the as-worked alloys of
this invention consists of a fine, elongated grain structure, with
a lath-like substructure and fine carbides, on the order of 0.01 to
0.03.mu.m in size, at the lath boundaries and dispersed within the
laths. This structure is also associated with a dominant
crystallographic texture of (111) ?110!, this designation is based
upon the well known Miller Crystallographic Index System. On rolled
plates this texture is measured on the transverse plane, or
perpendicular to the rolling direction. The intensity thereof
ranges from 2.5 to 3.5 times random as measured by the nine-plane,
(111) pole figure technique. A value of 1.0 represents a completely
random orientation, whereas 9.0 shows 100 percent, or the
orientation of a single crystal.
In contrast to this, the same alloy hot-rolled and finished above
about 1,600.degree. F. reveals a microstructure of large equiaxed
grains (ASTM No. 3-5). While there is no lath-like substructure,
large carbides, on the order of 1 to 5 .mu.m in size, are found
within the grains and at the grain boundaries. This texture will
nearly approach random with the intensity of any particular
orientation below 2.5 times random, by the system noted above.
The improved properties of the alloys of this invention can best be
demonstrated with rolling data and specific properties of six
bainitic alloys treated by the method herein, and by the
conventional hot rolling process, each of which are schematically
illustrated in the FIGURE. For convenience, the chemistry, by
weight percent, for the bainitic ferrous alloys are given in Table
II, with the rolling data and property results in Table III.
TABLE II
__________________________________________________________________________
Alloy C Mn P S Si Ni Cr Mo V Ti B
__________________________________________________________________________
AA .035 .79 .026 .022 .05 .02 .95 .97 .24 .006 -- AB .26 .77 .025
.015 .03 .04 .95 1.06 .25 .004 -- AC .46 .54 .016 .014 .15 1.86 .87
.26 .005 .003 -- AD .41 1.36 .015 .013 .33 .27 1.0 .46 .007 .001 --
AE .17 .59 .012 .012 .34 .02 .01 .52 .002 .001 .003 AF .17 .50 .012
.011 .25 .03 .01 .28 .001 .001 .003
__________________________________________________________________________
TABLE III ______________________________________ Properties Rolling
Temp. (.degree.F.) Transition Temp. Alloy Start Finish Y.S. (ksi)
(Cv15.degree. F) ______________________________________ AA1 1600
650 111 -180.degree. AA2 1600 700 120 -145.degree. AA3 1600 800 114
-80.degree. AA4 2200 1650 42 -90.degree. AB1 1530 620 198
-50.degree. AB2 1510 800 192 -200.degree. AB3 2140 1725 117
+75.degree. AC1 2080 1600 108 +200.degree. AC2 1800 990 106
-25.degree. AC3 1600 1175 115 -50.degree. AC4 1610 980 111
-75.degree. AD1 2020 1590 91 +200.degree. AD2 1790 995 104
-35.degree. AD3 1610 1175 108 -35.degree. AD4 1600 1000 109
-75.degree. AE1 2000 1600 45 -25.degree. AE2 1750 1000 103
-110.degree. AE3 1600 1190 96 -180.degree. AE4 1600 1000 113
-145.degree. AF1 2070 1600 46 -75.degree. AF2 1800 990 96
-100.degree. AF3 1650 1190 81 -130.degree. AF4 1615 1000 101
-170.degree. ______________________________________
To effect a proper comparison between the processed steel alloys of
this invention and the conventionally rolled steel alloys, all
reductions from 4 inches to 1/2 inch followed substantially one of
two schedules set forth in Table IV.
TABLE IV ______________________________________ Pass Sequence
Thickness (inches) Reduction (%)
______________________________________ 1 1a 3.5 12.5 2 2a 3.0 14.3
3 3a 3.0 (box pass) -- 4 4a 2.625 12.5 5 5a 2.125 19.0 6 6a 1.75
17.6 7 7a 1.375 21.4 8 8a 1.00 27.2 9 0.83 17.0 10 0.50* 40.0 9a
0.75 25.0 10a 0.625 16.7 11a 0.50* 20.0
______________________________________ *average elapsed time for
alloys finished above 1590.degree. F. was approximately 3 minutes;
the elapsed time for alloys finished between 1190-700.degree. F.
ranged between 10 and 26 minutes.
While temperatures were not ascertained following each pass for
each alloy, where measured they were found to be linear, following
a relatively constant cooling rate from the designated start to
finishing temperature. For the alloys of this invention, the
cooling rate varied between about 35.degree.-50.degree. F./min. The
foregoing is merely representative of reduction conditions, as
variations from this may be made, particularly with changes in
alloy composition. However, under all reduction practices, it is
essential that at least the final reduction takes place within the
bainitic region and concluded before the complete transformation of
the alloy to bainite.
Returning to the results of Table III, it will be evident that in
nearly all situations wherein the method of this invention was
followed, higher strength and improved impact properties were
realized. Thus, with the procedure detailed herein, it is now
possible to gain optimum mechanical and impact properties in
as-rolled steels without resorting to costly heat treatments.
The attainment of optimum properties is clearly demonstrated with
an exemplary comparison of Alloys AB3 and AB2. The yield strength
was increased by at least 64 percent while the transition
temperature was lowered from 75.degree. F. to -200.degree. F.
Another significant demonstration is noted when Alloy AB2 is
compared to Alloy AB1, the latter having been finish rolled after
complete transformation to bainite. While the precise critical
temperatures of the worked alloy are not known, such an alloy under
isothermal transformation conditions would reveal critical
temperatures for the B.sub.s and B.sub.f of approximately
920.degree. F. and 750.degree. F., respectively. The critical
temperatures for the worked alloy would be slightly higher.
Nevertheless, it is quite clear that even 750.degree. F., the
isothermal B.sub.f temperature, is well above the finish
temperature of 620.degree. F. for Alloy AB1. While the Y.S. thereof
remained at about the same level as the Y.S. for Alloy AB2, it will
be observed that a significant loss occurred in the transition
temperature. That is, the transition temperature was raised from
-200.degree. F. to -50.degree. F.
While it can be expected that improvement will vary with the
specific alloy being treated, it is believed significant that large
improvements in strength and/or impact properties were noted in all
situations.
* * * * *